While courts, protesters and politicians clash over abortion, medical science is quietly deciding the issue for many hospitals and physicians.

Abortion laws now hinge on the question of viability -- when a fetus can survive outside the womb. But medical advances are rapidly redefining viability.

This month, Memorial Hospital in Hollywood acquired a machine that allows full-term infants to breathe without using their lungs. Scientists think the device could be the first step toward creating an "artificial womb" that would allow embryos to survive almost from conception.

And that is causing some hospitals to rethink when they should perform abortions.

At least one local hospital, Plantation General, will no longer perform abortions past the 18th week of pregnancy because doctors feel uncomfortable terminating pregnancy after that date.

"If we're able to save a 500-gram (about 1 pound) baby, how can you think of abortion at that age?" asked Dr. Barry Chandler, head of neonatalogy at Plantation General. "We've pushed back the gestational age."

Florida, like most states, drew its abortion guidelines according to the viability of the fetus.

In state statutes, that is defined as the "stage of fetal development when the life of the unborn child may, with a reasonable degree of medical probability, be continued indefinitely outside the womb."

But that stage changes with each advance in medical technology. So, in effect, medical science is rewriting guidelines for abortion.

In 1973, when the Supreme Court upheld the legality of abortion in Roe vs. Wade, viability stood at about 28 weeks of gestation. Today, babies have survived after 22 weeks in the womb. Someday, doctors think, a baby could develop outside the womb.

"It definitely impacts abortion laws," said Katie Reinisch, interim executive director of Planned Parenthood of Miami. "Even those of us committed to pro-choice have a hard time saying we support late-term abortions. And as the point of viability creeps up ..."

In Florida, abortions cannot be performed past the 24th week of pregnancy unless the mother's health is in jeopardy. But already some doctors hestitate to perform an abortion at that stage.

"You have to be extremely careful because you can have a baby at 24 weeks that can survive," said Ron Goldberg, head of neonatalogy at Jackson Memorial Hospital. "It is causing people to be very cautious."

Doctors at Jackson will not perform abortions past 20 weeks. And abortion rights advocates say other hospitals and doctors may soon follow.

Anti-abortion activists say the new debate backs up their long-held stand that abortion should be illegal and not determined by viability.

"We should not let an artificial standard like viability determine whether a child lives or dies," said the Rev. Pat Mahoney of Operation Rescue. "If viability can be reduced to 15 weeks, 14 weeks, 10 weeks, will abortions be performed at all?"

The science that helps smaller, younger babies survive is a relatively new one.

The first neonatal specialists were licensed in the early '70s, a time when 95 percent of infants born weighing 750 grams (1 lb., 10.5 oz.) or less did not survive. In 1991, only 9 percent of babies that small at Jackson Memorial Hospital died.

Infants born before 37 weeks of gestation are prone to a dizzying range of medical setbacks. Their lungs are underdeveloped and easily damaged. Their skin is thin and suspectible to infections.

Their eyesight and hearing are ultrasensitive. They can fall prey to bleeding in the brain. They are more likely to develop cerebral palsy and learning disabilities.

Scientists have designed ventilators for them. They have developed formulas and feeding methods for them. But the greatest advance so far has been the development of artificial surfactant -- the lubricant that helps lungs stay inflated. Because preemies are born without surfactant, their air sacs deflate totally with each breath.

However, an even greater breakthrough is on the horizon.

The ECMO machine, the heart-lung bypass device that made its debut at Memorial Hospital in Hollywood last week, could be a link to those discoveries.

The machine takes blood out of the veins through a tube, removes the carbon dioxide, then pumps oxygen back into the body, said Dr. David Drucker, a pediatric surgeon.

However, to keep the blood from clotting, ECMO patients are given an anti- clotting drug, which preemies are too small to take.

As it exists today, ECMO can only be used on babies of at least 36 weeks' gestation who weigh a minimum of 4 pounds. Yet researchers already are trying to find ways to adapt it for preemies.